1. Technical Field
The present invention relates to a deterioration detector for an exhaust gas sensor to detect a concentration of oxygen contained in exhaust gas discharged from an internal combustion engine and, more particularly, to a deterioration detector for an exhaust gas sensor capable of totally detecting deterioration of an exhaust gas sensor and abnormal condition of an electric heater wherein an exhaust gas sensor used in combination with an electric heater thereby improving accuracy in detection of an exhaust gas is employed. The invention relates also to a method for detecting the deterioration.
2. Background Art
A conventional technique has been well known in which a concentration of oxygen contained in exhaust gas discharged from an internal combustion engine is detected, feedback control of an air/fuel ratio of fluid mixture supplied to the internal combustion engine is conducted and exhaust gas is purified and a rate of fuel combustion is improved.
In order to conduct the feedback control, it is necessary to keep the exhaust gas sensor within an activating region and stabilize detection characteristic of the oxygen concentration. Therefore, a ceramic heater incorporated in the exhaust gas sensor is controlled in terms of energizing and the exhaust gas sensor is kept at a constant temperature.
As to detection of a temperature of an exhaust gas sensor to serve as a basic art, for example as disclosed in the Japanese Patent Publication (unexamined) No. 292364/1997 and the Japanese Patent Publication (unexamined) No. 313477/1996, a method is employed wherein an internal resistance of an exhaust gas sensor is measured and temperature of the exhaust gas sensor itself is detected using the internal resistance of the exhaust gas sensor as a reference. Additionally, as disclosed in the Japanese patent Publication (unexamined) No. 313476/1996 and the Japanese patent publication (unexamined) No. 4502/1997, such a method is employed as to detect a peripheral temperature of an exhaust gas sensor by measuring a resistance value of an electric heater incorporated in the exhaust gas sensor.
As for a method of detecting deterioration of an exhaust gas sensor, as disclosed in the Japanese Patent Publication (unexamined) No. 331700/1998 and the Japanese Patent Publication (unexamined) No. 121221/1996, means for changing intentionally operating condition of an internal combustion engine and observing reaction change speed and output variance waveform of a detection output of an exhaust gas sensor in response to the change of operating condition of the internal combustion engine.
In case of employing such a conventional art as described above, a problem exists in that large-scaled means for changing operating condition of the internal combustion engine has been required just for detecting the deterioration of the exhaust gas sensor. A further problem exists in that abnormal condition of the electric heater integrated with the exhaust gas sensor cannot be detected.
The present invention was made to solve the above-discussed problems, and has an object of providing a deterioration detector for an exhaust gas sensor and a method for detecting deterioration wherein total deterioration detection of an exhaust gas sensor including deterioration of an electric heater is easily performed simply by utilizing means for observing change with age in internal resistance of an exhaust gas sensor together with detection of abnormal condition of the electric heater.
The present invention is to provide a simple deterioration detector for an exhaust gas sensor and a method for detecting deterioration wherein change with age in internal resistance is extracted taking into consideration fluctuation between products and temperature dependency of an exhaust gas sensor and an electric heater.
A deterioration detector for an exhaust gas sensor according to the present invention that is mounted on an exhaust pipe of an internal combustion engine for an automobile and is arranged so as to control a temperature by an electric heater thereby detecting deterioration of the exhaust gas sensor comprises:
initial internal resistance correlation learning storage means for sequentially sampling, comparing, computing and storing an initial internal resistance of said exhaust gas sensor and said electric heater;
data collection period judgement means for determining a period of sampling said initial internal resistance; and
abnormal condition judgment means for computing and storing a current internal resistance of said exhaust gas sensor and said electric heater after a predetermined period of time of an operation start of said internal combustion engine for an automobile, for judging that said current internal resistance has varied greatly over a predetermined permissible value by comparing said current internal resistance with said initial internal resistance, thereby conducting a processing for the abnormal condition.
As a result of such an arrangement, deterioration of the exhaust gas sensor as well as the abnormal condition of the electric heater can be detected, and a timing of replacement and repair of an integrally formed exhaust gas sensor and electric heater can be known.
In the deterioration detector for an exhaust gas sensor of above arrangement according to the invention, it is preferable that, in the case that an initial internal resistance of the exhaust gas sensor and the electric heater which is newly sampled, computed and measured is closely analogous to the initial internal resistance already learned and stored, the initial internal resistance correlation learning storage means averages the value of said initial internal resistance newly sampled and that of said initial internal resistance learned and stored, and stores the averaged value for renewal.
As a result of such arrangement, any useless proximate information is not stored, a memory capacity can be reduced, and an amount of information to be referenced and read out is eliminated, making it possible to perform a high-speed processing.
Further, in a deterioration detector for an exhaust gas sensor of above arrangement according to the invention, it is preferable that the data collection period judgement means judges a completion of the data collection period based on the fact that a sampling number of the initial internal resistance of the exhaust gas sensor or the electric heater stored in initial internal resistance correlation learning storage means, is not less than a predetermined number and a correlation coefficient not less than a predetermined value is surely obtained.
As a result of such arrangement, even if there might be any fluctuation of the internal resistance of the exhaust gas sensor or the electric heater between one sensor or heater and another, learning and storing an initial value of an actually utilized sensor or heater enables to detect a later change, and a data collection period can be determined by confirming whether or not a sufficient number of data could be obtained.
Further, in a deterioration detector for an exhaust gas sensor of above arrangement according to the invention, it is preferable that the abnormal condition judgment means comprises computing interpolation means for computing, on the assumption that either one of the current internal resistance of the exhaust gas sensor or the electric heater is an initial internal resistance, the other correlation initial internal resistance, comparing the other correlation initial internal resistance with the other current internal resistance, and producing an abnormal condition judgement output in the case of a difference between the other correlation initial internal resistance and the other current internal resistance being greatly varied over a predetermined permissible value.
As a result of such arrangement, even if the internal resistance of the exhaust gas sensor and the electric heater has greatly changed due to an environmental temperature, it can be discriminated whether the current internal resistance has changed with age or has changed due to environmental temperature. An accurate correlation value can be calculated even though there might be a small amount of stored information, making it possible to reduce a memory capacity.
It is also preferable that the deterioration detector for an exhaust gas sensor of above arrangement according to the invention, in addition to the arrangement as described above, further comprises reset means for initializing information stored in each of data collection period judgement means, initial internal resistance correlation learning storage means and abnormal condition judgement means in response to a reset signal from outside.
As a result of such arrangement, even in the case of replacing the deteriorated exhaust gas sensor with a new one, detection of deterioration of such a new exhaust gas sensor can be conducted under the condition suitable for the new exhaust gas sensor.
A method for detecting deterioration of an exhaust gas sensor which is mounted on an exhaust pipe of an internal combustion engine for an automobile and arranged so as to control a temperature by an electric heater thereby detecting the deterioration of the exhaust gas sensor, comprising the steps of:
computing and storing an initial internal resistance of said exhaust gas sensor and said electric heater;
learning and storing a correlation in initial internal resistance between said exhaust gas sensor and said electric heater;
computing and storing a current internal resistance of said exhaust gas sensor and said electric heater;
calculating, on the assumption that either one of the current internal resistance of said exhaust gas sensor or said electric heater is an initial internal resistance based on said correlation learned and stored, the other logical internal resistance; and
detecting a deterioration condition of said exhaust gas sensor and said electric heater by comparing the logical internal resistance with the other current internal resistance.
As a result, deterioration of the exhaust gas sensor as well as the abnormal condition of the electric heater can be detected, and a timing of replacement and repair of an integrally formed exhaust gas sensor and electric heater can be known.
Furthermore, even if the internal resistance of the exhaust gas sensor and the electric heater has greatly changed due to an environmental temperature, it can be discriminated whether the current internal resistance has changed with age or has changed due to environmental temperature. An accurate correlation value can be calculated even though there might be a small amount of stored information, making it possible to reduce a memory capacity.